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The structural evolution (not only intra-fibrillar but also inter-fibrillar structures) during extension of poly (vinyl alcohol) (PVA) in water at different temperatures are investigated by in-situ small- and wide-angle X-ray scattering (SAXS and WAXS). The stress-strain curves can be defined into four zones with the onset and the end of stress plateau, and the strain hardening point as the zone boundaries. WAXS results show that stretch induces melting of PVA crystal from zones I to III, while SAXS measurements reveal stretch-induced transformation from initial lamella to nanofibrils starts at the onset of zone II, which proceeds via a melting-reconstruction process instead of direct transformation. In zone III, the nanofibrils increase in content and pack periodically with an average inter-fibrillar spacing of about 14–18 nm. Increasing temperature leads to larger inter-fibrillar spacing and more perfect packing of nanofibrils, which may benefit the production of polarizer with homogeneously structural and optical properties. [ABSTRACT FROM AUTHOR]

Combining a homemade uniaxial stretching device and the simultaneous in-situ synchrotron radiation WAXS measurement, the structural evolution during transverse direction (TD) stretching of machine direction (MD) pre-stretched Poly(ethylene terephthalate) (PET) film has been investigated. Current work reveals that increasing the MD pre-stretch before the occurrence of stretch induced crystallization (SIC) can enhance the crystallization and orientation in TD stretching. The concept of parallel chains structures (PCS) cross-linked network is proposed to interpret the rubber-like mechanical behavior. An appropriate stretch ratio window in stretching above the glass transition temperature (T g) is existed, where the PCS cross-linked network before crystallization can be induced by stretching above T g. With the help of the PCS cross-linked network, the chain orientation is retained more effectively under stretching, promoting the formation of subsequent crystallization and crystal cross-linking network. The strain hardening in different pre-stretched samples occurs accompanied by the same crystallinity and various chains orientation. This demonstrates that the strain hardening is determined by the crystal cross-linked network rather than the orientation. Current work supplies a guidance for getting the improved film products of PET with enhanced comprehensive mechanical properties, process stability and higher chain orientation. • The structural evolution in subsequent stretching of pre-stretched Poly(ethylene terephthalate) (PET) film is investigated by the in-situ synchrotron radiation WAXS measurement. • The origin of the novel rubber-like mechanical behavior under the complicated stretching field is better understood by the combination of the in-situ chains orientation, crystallization behavior and mechanical behavior. • The concept of parallel chains structures (PCS) cross-linked network is proposed to interpret the rubber-like mechanical behavior. • The onset of strain hardening corresponds to the formation of the crystal cross-linked network. [ABSTRACT FROM AUTHOR]

The evolution of poly (vinyl alcohol) PVA structure and dichromatic substance induced by uniaxial stretching PVA in the mixed solution with iodine and boric acid (BA) is systematically studied by the combination of in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXD). The experiment is carried out under an orthogonal experimental design for the iodine concentrations of 0.01, 0.2, 1 M, and the BA concentrations of 0.3, 1, and 3 wt%, respectively. We divide the stretch process into three regions, whose boundary is the yield point and strain harden point. The PVA-I 3 - co-crystal generating at high and moderate iodine concentrations is less stable as compared with the PVA crystal, which leads to the earlier decrease of crystallinity at the end of the first region. In the second region, the broken crystal, namely the regular amorphous, is the precursor of the PVA-polyiodide complex. Further increasing the strain, the PVA-I 3 - co-crystals are generated from the one dimension polyiodide surrounded by the PVA chains through a melt-reconstruction process. The adequate BA can make it easy to form the polyiodide at the initial state. Still, the increasing rate of polyiodide is suppressed by further added BA due to the limitation of chain movement by the crosslinking agent. In the final region, the emergence of nanofibril can account for the strain hardening of the film. The evolution of polyiodide and PVA-I 3 - co-crystal revealed by this work gives a better understanding of the polarizer production process, which may guide the resolution of blue leakage of polarizer via an enhancement of the I 3 − in the crystal form during stretching. Image 1 • The structural evolution of dichromatic substance with poly (vinyl alcohol) at different concentrations of boric acid during stretch is investigated by in-situ WAXD and SAXS. • The metastable PVA-I 3 - cocrystal will be easily broken during stretching, which leads to the distinct yield point. • Polyiodide first emerges in the amorphous originated from the broken crystal and then in the highly oriented amorphous during stretching. • The one-dimension polyiodide along with the PVA chain is the precursor of PVA-I 3 - cocrystal. • Dichromatic substance emerges earlier with the increasing of boric acid concentration; however, their final content is less due to the limitation of chain mobility by the crosslinking agent. [ABSTRACT FROM AUTHOR]